1. Field of the Invention
The present invention relates to a display in which each pixel is provided with a light emitting element, the luminance of which is controlled by a current such as an organic electroluminescent (EL) element. More specifically, the present invention relates to an active matrix display for supplying a current to a light emitting element by an active element such as an insulated-gate field-effect transistor provided inside each pixel.
2. Related Background Art
In recent years, displays using an organic EL element have been developed. As a method of driving the element, there are a simple matrix system and an active matrix system. Since the former is simple in its structure but has difficulty in realizing a large and high definition display, many active matrix type displays have been developed.
If a large number of organic EL elements are used and driven by an active matrix circuit, an insulted-gate field-effect transistor, a so-called thin film transistor (hereinafter referred to as TFT), for controlling supply of a driving current for driving a light emitting element, is connected to each pixel. A light emitting operation of the organic EL element is controlled by controlling this TFT.
Background Example 1
FIG. 9 shows an equivalent circuit for one pixel disclosed in U.S. Pat. No. 5,684,365.
A pixel circuit provided in a pixel is constructed by an organic EL element OLED, a thin film transistor (TFT) 1, a thin film transistor (TFT) 2 and a capacitor C. Since an organic EL element generally has a rectification characteristic, it is sometimes called an organic light emitting diode (OLED). In the figure, a symbol of a diode is used. However, a light emitting element is not always limited to the OLED, but also may be any light emitting element as long as its luminance is controlled by a current flowing to the element. In addition, the rectification characteristic is not always required. In FIG. 9, a source and a drain of the p-type TFT 2 are connected to a power supply potential Vdd and an anode of the organic EL element OLED, respectively, and a cathode of the organic EL element OLED is connected to a ground potential. On the other hand, a gate, a source and a drain of the p-type TFT 1 are connected to a scanning line Scan, a data line Data, and one end of the capacitor C, and a gate of the TFT 2, respectively, and the other end of the capacitor C is connected to the power supply potential Vdd.
First, when the TFT 1 is turned ON by the scanning line Scan to apply a data potential Vw representing luminance information to the data line Data in order to operate the pixel, the capacitor C is charged or discharged, whereby a gate potential of the TFT 2 becomes equal to the data potential Vw. When the TFT 1 is turned OFF by the scanning line Scan, the gate potential of the TFT 2 is held by the capacitor C, and a driving current corresponding to a gate to source voltage Vgs of the TFT 2 is supplied to the organic EL element OLED. Thus, the organic EL element OLED continues to emit light at a luminance corresponding to an amount of the current.
Background Example 2
FIG. 10 shows an equivalent circuit for one pixel disclosed in JP 2001-56667 A.
A pixel circuit provided in a pixel is constructed by an organic EL element OLED, a TFT 1 for converting a signal current to a voltage or supplying a current to the organic EL element OLED, a TFT 2 for controlling an operating state of the TFT 1, a TFT 3 and a TFT 4 for selecting a state in which a signal current is taken in or a state in which a driving current is supplied to the organic EL element OLED, and a capacitor C for holding a voltage.
In FIG. 10, a source and a gate of the TFT 1 are connected to a power supply potential Vdd, and a source of the TFT 2 and one end of the capacitor C, respectively. The other end of the capacitor C is connected to the power supply potential Vdd. A drain of the TFT 1 is connected to a drain of the TFT 2, a drain of the TFT 3 and a drain of the TFT 4. A source of the TFT 4 is connected to an anode of the organic EL element OLED, and a cathode of the organic EL element OLED is connected to a ground potential. A source of the TFT 3 is connected to a data signal line Data, and all gates of the TFT 2, TFT 3 and TFT 4 are connected to a scanning line Scan.
First, when the TFT 2 and the TFT 3 are turned ON and the TFT 4 is turned OFF by the scanning line Scan in order to operate the pixel, a signal current Iw is taken in the TFT 1, a gate to source voltage Vgs required for flowing the signal current Iw is generated in the TFT 1, and the voltage Vgs is held in the capacitor C. When the TFT 2 and the TFT 3 are turned OFF and the TFT 4 is turned ON by the scanning line Scan, the TFT 1 continues to flow a driving current to the organic EL element OLED based on the voltage held in the capacitor C. Thus, the organic EL element OLED continues to emit light at a luminance corresponding to an amount of the current.
Background Example 3
FIG. 11 shows an equivalent circuit for one pixel disclosed in JP 2001-147659 A (EP A2 1102234).
A pixel circuit provided in a pixel is constructed by a TFT 1 for converting a signal current to a voltage, a TFT 2 for controlling a driving current flowing to a light emitting element, a TFT 3 for taking in a current which connects or disconnects the pixel circuit and a data line by a scanning line ScanA, a transistor for switching TFT 4 that shorts between a gate and a drain of the TFT 1 while luminance information is written by a scanning line ScanB, a capacitor C for holding a gate to a source voltage of the TFT 1 even after the luminance information is written, and an organic EL element OLED.
In FIG. 11, sources of the TFT 1 and the TFT 2 are connected to a power supply potential Vdd, and a gate of the TFT 1 is connected to a gate of the TFT 2, one end of the capacitor C and a drain of the TFT 4. The other end of the capacitor C is connected to the power supply potential Vdd. A drain of the TFT 2 is connected to an anode of an organic EL element OLED, and a cathode of the organic EL element OLED is connected to a ground potential. A drain of the TFT 1 is connected to a source of the TFT 4 and a drain of the TFT 3. A source of the TFT 3 is connected to a data signal line Data. A gate of the TFT 3 is connected to a scanning line ScanA, and a gate of the TFT 4 is connected to a scanning line ScanB.
First, when the TFT 3 and the TFT 4 are turned ON by the scanning lines ScanA and ScanB in order to operate the pixel, the TFT 1 and the TFT 2 come to have a current mirror structure. A signal current Iw is taken in the TFT 1, the TFT 2 flows a current to the organic EL element OLED in accordance with a current mirror ratio, and a voltage generated in the gate of the TFT 1 is held in the capacitor C. When the TFT 3 and the TFT 4 are turned OFF by the scanning lines ScanA and ScanB, the current mirror structure of the TFT 1 and the TFT 2 is released. The TFT 2 continues flowing a current to the organic EL element OLED in accordance with the voltage held in the capacitor C. The light emitting element continues to emit light at a luminance corresponding to an amount of the current.
In an active matrix display, thin film transistors functioning as active elements are generally formed on a single glass substrate simultaneously using amorphous silicon or polysilicon. However, the TFTs that are formed using amorphous silicon or polysilicon are known to have large variation of their characteristics, because the TFTs have worse crystallinity and worse controllability of a transmission mechanism compared with monocrystal (single crystal) silicon.
Therefore, it is not rare that, even in the TFTs formed on the same substrate, their threshold voltages Vth vary by several hundred mV or, in some cases, 1V or more for each pixel. In this case, for example, since the Vth varies depending on a pixel even if the same signal potential Vw is written in different pixels, a current flowing to a light emitting element changes, and a desired luminance cannot be obtained. Therefore, a high image quality cannot be expected as a display.
The structure of Background Example 1 (U.S. Pat. No. 5,684,365) is directly affected by this problem. In addition, Background Example 2 (JP 2001-56667 A) solves the problem of threshold voltage variation. However, since a source/drain voltage Vds of the TFT 1 at the time when a signal current is converted into a voltage and a source/drain voltage Vds of the TFT 1 at the time when a driving current is supplied to the organic EL element OLED are different, a correct driving current based on a data signal cannot be flowed to the light emitting element due to the Early effect of a transistor. In addition, the Background Example 3 (JP 2001-147659 A) changes variation of threshold voltages to error levels of the current mirror constructed by the TFT 1 and the TFT 2, thereby reducing the variation. However, it does not fundamentally solve the problem. Further, since a source/drain voltage Vds1 of the TFT 1 is different from a source/drain voltage Vds of the TFT 2, an accurate driving current cannot be flowed to the light emitting element due to the Early effect of a transistor as in Background Example 2. Moreover, if an operating voltage of the organic EL element OLED increases and the source/drain voltage of the TFT 1 cannot be secured sufficiently with the result that the transistor operates in a triode region, a current deviating largely from a desired driving current is supplied to the light emitting element.
The present invention has been devised in view of the above-mentioned drawbacks, and it is an object of the present invention to provide an active matrix display that solves the problem associated with variation of driving current to be supplied to a light emitting element, which is attributable to variation of a threshold voltage present in the above-mentioned conventional techniques and that is higher in performance than conventional displays.
Therefore, according to the present invention, there is provided an active matrix display in which a plurality of pixels provided with a pixel circuit containing at least a light emitting element are arranged in a matrix shape and which has at least a scanning side drive circuit and a data side drive circuit for performing control of the pixel circuit, wherein the light emitting element is a light emitting element of a current control type, the luminance of which changes according to a driving current flowing to the light emitting element, wherein the pixel circuit comprises at least the light emitting element, a first voltage control current source, a first switch circuit, a driving current-voltage converter, a second voltage control current source and a second switch circuit, the first voltage control current source comprising at least an active element controlled by a control voltage and a memory circuit capable of storing the control voltage and having a function of generating the driving current based on the control voltage, the first switch circuit having a function of switching the first voltage control current source to a voltage controllable state and a control voltage holding state, the driving current-voltage converter being serially connected to a current path through which the driving current flows and having a function of converting the driving current into a voltage, the second voltage control current source having a function of generating a monitor current correlating with the driving current based on an output voltage of the driving current-voltage converter, and the second switch circuit having a function of switching the second voltage control current source to an output state and a non-output state, wherein the scanning side drive circuit is at least connected to the first switch circuit and the second switch circuit and has a function of performing control for switching the first voltage control current source to the voltage controllable state or the control voltage holding state and control for switching the second voltage control current source to the output state or the non-output state, and wherein the data side drive circuit is at least connected to the first voltage control current source via the first switch circuit and connected to the second voltage control current source via the second switch circuit and has a function of controlling a control voltage of the first voltage control current source based on the monitor current correlating with the driving current such that a current value of the driving current becomes a desired current value corresponding to luminance information when the first voltage control current source is in the voltage controllable state and the second voltage control current source is in the output state.
Further, a voltage control current source indicates means for regulating a current that is flowed based on a voltage, a driving current-voltage converter indicates means for outputting a voltage correlating with a driving current, a monitor current-voltage converter indicates means for outputting a voltage correlating with a monitor current, a voltage comparator indicates means for not only comparing voltages but also for outputting a voltage based on the comparison.
In addition, a voltage controllable state indicates a state in which it is possible to change and control a control voltage, a control voltage holding state indicates a state in which a control voltage recorded in a storage circuit is not allowed to be changed from the outside, an output state indicates a state in which a monitor current is allowed to flow, and a non-output state indicates a state in which a monitor current is not allowed to flow.
Other objects and features of the present invention will become apparent from the following detailed description and accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof.